The present invention relates to improvements in glass melting furnaces. In particular, the invention relates to improvements in controlling the melting of batch materials in a cold-crown or vertical glass melting furnace.
Various problems are encountered in controlling the melting of cold-crown glass melting units which are not present in the standard fuel-fired hot crown melting furnace. That is, the usual glass melting tank of the prior art had a free surface of molten glass, with batch material being supplied at one end as shown in U.S. Pat. No. 3,208,841. Heat was supplied to melt the batch material on the surface of the molten glass through the use of combustible fuel supplied over the surface thereof and/or through the use of electrodes immersed within the molten bath. In either case, the tank was provided with a roof over the free surface and the firing of the combustible fuel over such surface provided for a hot crown. The firing of the combustible fuel and/or the immersed electrodes, provided energy to melt the batch material supplied at one end before being removed from an opposite end of the furnace.
In a cold-crown vertical melting furnace, such as shown in U.S. Pat. No. 4,429,402, a layer of batch material is provided upon and completely covers the surface of molten glass retained within the glass melter. The layer of batch serves both to contain the heat energy release adjacent the top of the molten glass and to provide the glass making materials necessary for melting. Electric power is introduced by means of electrodes which preferably extend through the batch layer and into the molten glass, and the melted glass is withdrawn from a central bottom portion of the furnace for delivery to a desired location.
Typically, the pull rate on a standard fuel-fired furnace is dictated by the forming process requirements. The fill rate is thus controlled to hold a target glass level in the distribution zone of the furnace. Since the glass level in such furnaces is actually a free surface, detectors are readily available to determine the level or fusion line of the glass in the distribution zone, and accordingly closed loop control to the furnace filling machines is a fairly simply process to implement. However, in cold-crown vertical melters, there is no free glass surface which can be easily measured. Further, it has been found that the height or thickness of the batch material on the surface or fusion line of the molten glass materially affects the resultant quality of glass produced by the melter, particularly when the batch material is composed of fairly fine raw materials in the range of about 300 mesh or finer.
Accordingly, the present invention sets forth a novel method of controlling the melting of batch materials in a cold-crown furnace by determining and controlling the height of the batch material maintained above the fusion line, i.e. the thickness of the batch blanket.